- Email: [email protected]
Laser Assisted Joining of Metal Pins and Thin Plastic Sheets
Available online at www.sciencedirect.com
Physics Procedia 39 (2012) 108 – 116
LANE 2012
Laser assisted joining of metal pins and thin plastic sheets Andor Bauernhuber , Tamás Markovits Budapest University of Technology and Economics, Department of Vehicles Manufacturing and Repairing, 1111 Budapest, Bertalan Lajos u. 2, Hungary
Abstract The joining of plastics and metals in order to produce lightweight but robust parts has become more significant in the process of vehicle manufacturing. In the course of this study, the authors joined PMMA plastic and steel in a pin to plate geometry by pulsed Nd:YAG laser. Tensile tests were carried out to investigate the effects of heating time, the laser settings, the surface roughness and the clamping force on the joining strength. Experimental results showed that the joining is feasible and using adequate settings the strength can be optimised. © © 2012 2011 Published Published by by Elsevier Elsevier B.V. Ltd. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Keywords: laser; steel; PMMA; hybrid; joining
1. Introduction Growing demand for new material combinations from the side of the industry, like plastic and metal combinations, requires new methods, also in the area of joining technologies. The fact that plastics are well applicable in technical structures accelerates the degree of their utilization, thus it is necessary to elaborate the procedure of their joining to other structural materials, like metals. The technologies applied so far have many disadvantages: in case of mechanical fasteners the stress peak around the joint and the difficult automation, while in case of the most widely used adhesives the long bonding time and the harmful volatile compounds cause difficulties [1, 2]. A possible solution for the listed problems may be provided by applying the laser beam directly to join metal and plastic, like it is already used to join the same kind of materials (metal to metal, plastic to plastic) [3, 4]. The aim of the research was to investigate the factors influencing the bond properties, and to describe their effect on the joint strength in pin to plate geometry. It was investigated how the heating time, the
* Corresponding author. Tel.: +36-146-31-838 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH doi:10.1016/j.phpro.2012.10.020
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
clamping force, the surface roughness and the laser settings, like average power, pulse frequency, pulse time and pulse energy affect the joint strength and the penetration of the steel work piece into the plastic. 2. Experimental In the experiments, steel pin samples were bonded with plastic sheets. The steel material was S235, the plastic material was poly methyl metacrylate (Acriplex PMMA-XT), the geometry of steel sample and the schematic view of the experimental setup can be seen in Fig. 1, the size of the plastic sheet was 15 x 15 x 2 mm. This plastic was chosen because of its good strength results during the preliminary experiments and because of its transparency: the structural changes in the plastic and the connecting area could be observed easily. The laser beam source was a LASAG SLS 200 type, pulse mode Nd:YAG laser with a maximal pulse power of P max =5.5 kW and with an average power of P a =220 W. The power distribution of the laser beam was Gaussian (TEM 0,0 ).
Fig. 1. Schematic view of the experimental setup
The sample was irradiated from the plastic side. The plastic is highly transparent (90 %) to the laser beam [5], so the beam was transmitted through the plastic and was mainly absorbed by the steel surface. The steel was heated directly and the face surface transferred the heat to the plastic. The plastic became softer and finally melted due to the heat input. The softened material flowed back along the lateral surface of the pin and formed a burr ring at the entrance hole. At the end of the process, the steel pin penetrated into the sheet and was surrounded by a burr ring. After cooling, the joint was created. Investigating the heating time, it was changed from 3 s to 7 s, the applied laser settings were the following: f = 100 Hz, tp = 0,5 ms, E = 2 J where f is the pulse frequency, tp is the pulse duration in time, E is the pulse energy, the value of clamping force was 3.2 N. To examine the compressive force, the force values were 3.2 N, 6 N and 9.2 N. The heating time was 4 s. Investigating the surface roughness, the heating time was 6 s. The average surface roughness changed from R a =0,5 μm to R a =10 μm. Examining the laser settings, the heating time was 4s; the value of the clamping force was 3.2 N. The applied laser settings were the following: the frequency was changed between 4 Hz and 200 Hz, pulse time was varied between 0.3 ms and 9.9 ms, and the pulse energy was changed between 1 J and 40 J. In the first case, the effect of average power was investigated, the average power was varied from 120 W to 200 W by adjusting the pulse frequency. In the second case, constant pulse energy of 2 J, in the third case nearly constant pulse power of 4000 W ± 400W, in the fourth case constant pulse time of 2 ms was applied. The average power was 200 W in the last three cases. In each case 4.75 l/min argon shielding gas was applied and the average surface roughness of the steel pins on the lateral surface was altered between 0.5 μm and 2.5 μm except at investigating the effect of surface roughness. The pins were manufactured by turning. Before the experiment the steel pins were cleaned with acetone.
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To investigate the bonding force, the workpieces were torn 24 hours later. In the results section the force values are given because the determination of the bonding area was not obvious. Therefore the maximal tearing force was used to characterise the strength of the joint. 3. Results and Discussion 3.1. Effect of heating time and clamping force First, the effect of heating time (t h ) on the penetration depth and the tearing force was investigated. Our aim was to determine the process window in which the joint is feasible. The shortest time when a joint could be created was 3 s. A shorter heating time did not ensure the needed heat amount to melt the plastic, there was not a sufficient penetration. The longest time was 7 seconds, because at a longer heating time the steel pin perforated the plastic sheet, so that the measured penetration became bigger than 2 mm. Therefore the applied heating time values were 3, 4, 5, 6 and 7 seconds. In Fig. 2 (a) and (b), the photo of side and the top view of a joint, in Fig. 3 (a) and (b), the effect of heating time on penetration depth and maximal tearing force are shown at 3 different clamping force values.
F clamping = 3.2 N a)
F clamping = 6 N b)
F clamping = 9.2 N
Fig. 2. Photos of the joint at different clamping force values: (a) side view; (b) top view of the created joint and bubble formation
a)
b)
Fig. 3. Effect of heating time at different clamping force: (a) on penetration; (b) on maximal tearing force
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
In Fig. 3 (a) it can be seen, that in the case of a longer heating time the penetration increases: the temperature of the plastic rises in the vicinity of the steel, and the material softens. The softened material flows back next to the steel surface so that continually new plastic material melts in front of the heated pin. Accordingly, the longer the heating time, the deeper the steel pin penetration into the plastic due to the operating clamping force. The maximal tearing force can be seen in Fig. 3(b) against the heating time. The increasing rate of tear force shows a similar tendency with the results in Fig. 3 (a). To increase the maximal tearing force the joint has to be heated longer: the average tear force value changes from 150 N to 230 N. The explanation of the phenomenon is the deeper penetration: the contact surface grows in the case of a longer heating, resulting in a higher maximum force. If we compare the results of Fig. 3 (a) and Fig. 3 (b) at 3.2 N clamping force, we can see that the growth rate of the maximal tearing force has a slowing tendency with the heating time while the penetration grows in an accelerating way, creating a larger contact surface. The difference can be explained by the formation of bubbles: at longer heating times the temperature of the melted plastic increases, and it probably also decomposes, forming many bubbles at the steel surface. The bubbles weaken the joint because they separate the base plastic material from the steel. The bubble formation is more intensive in the case of longer heating times so their effect occurs primarily at the longer heating time range (Fig. 4 (a)). The effect of bubbles can be observed after the tearing of the joints as well. At low heating times the tearing occurs in many cases in the base plastic material and large parts of plastic remain on the face surface. At long heating times only a thin layer of plastic remains on the surface of the pin including the imprints of the created bubbles. The effect of heating time on the bubble formation and the surface of the pins after tearing can be seen in Fig. 4.
t heating = 3 s
a)
t heating = 7 s
t heating = 4 s
b)
t heating = 7 s
Fig. 4. Effect of heating time (a) on bubble formation; (b) on type of tearing
In Fig. 3 (a) and in Fig. 3 (b) the effect of clamping force on the measured penetration and maximal tearing force is plotted as well. As expected, the higher clamping force causes deeper penetration, proportional to the force value: the character of the curves is the same, they run in parallel. At clamping forces of 6 N and 9.2 N the maximal heating time is only 6 s, because the pin perforates the plastic sheet already at 7 s of heating time. The evolution of the curves in Fig. 3 (b) is not as obvious as in Fig.3 (a). The curves measured at different clamping forces are moving together at low heating times of 3 s and 4 s. At longer heating the curves split, and the higher clamping force leads to a higher tearing force. The phenomenon can be explained with the bubble formation again. Comparing the size and amount of bubbles at different clamping forces, we can see, that the bubble formation is less intensive at higher clamping forces: the higher pressure hinder the growing of bubbles and pushes them into the burr, as showed in Fig. 2 (b). Therefore the area of the bubbles can be regulated with the clamping force value and the result is a better contact and a higher tearing force. However, if too high pressure is used, the plastic sheet can be perforated without reaching the temperature needed for a good adhesion.
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3.2. Effect of surface roughness The effect of surface roughness on the maximal tearing force and on the penetration is showed in Fig. 5 (a) and (b). In this case a heating time of 6 s was chosen to reach a higher penetration, and to ensure a larger contact area on the lateral surface to. The tearing force value can be enhanced strongly by increasing surface roughness. On the one hand the soft and molten plastic takes the shape and flows into the valleys of the steel surface generating a mechanical interlocking. On the other hand the higher surface roughness improves the absorption of laser radiation at the top surface. This means a higher temperature at the same heating time, and so this leads to a deeper penetration. This increased penetration allows a larger contact area and makes possible for more roughness peaks to be in contact with the plastic material. The characteristic cross sections of the joints can be seen in Fig. 6: the roughness peaks and the bubbles on the surface are well identifiable.
a)
b)
Fig. 5. Effect of surface roughness (a) on penetration depth; (b) on maximal tearing force
a) Ra = 1 μm
b) Ra = 10 μm
Fig. 6. Cross section of the created joint at lateral surface (a) Ra=1 μm; (b) Ra=10 μm
3.3. Effect of laser settings In Fig. 7 (a) and (b) we can see the effect of average laser power on the penetration and on the tear force values. The power was adjusted by using different pulse frequencies while pulse energy and pulse time was unchanged. In both cases a similar tendency can be discovered as in Fig. 3 (a) and (b): the average power has an effect like the heating time. At least a power of 120 W is needed to create a joint. With increasing average power the penetration grows linearly, while the tear force has a slowing growth tendency. The difference is caused by the bubble formation, as mentioned in the section describing the heating time. In Fig. 8 (a) and (b) we can see the effect of pulse power on the penetration and maximal tearing force, while the pulse energy and pulse frequency was constant, E p = 2 J and f p = 100 Hz, respectively. Pulse
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
power was changed between 4.46 kW and 0.64 kW and pulse time was changed between 0.45 ms and 3.14 ms. In the diagram, the value of pulse time is given at each setting. In Fig. 9 (a) and (b) the effect of pulse power on the penetration and maximal tearing force is showed, while the pulse time was constant, t p =2 ms, and the pulse energy and pulse frequency was adjusted. Pulse energy was changed between 2 J and 10 J, pulse power was changed between 4.7 kW and 1 kW and pulse frequency was changed between 22 Hz and 100 Hz. In the diagram, the value of pulse energy is given at each setting.
a)
b)
Fig. 7. Effect of average power: (a) on penetration depth; (b) on maximal tearing force
Ep = 2 J fp = 100 Hz Pa = 200 W
a)
Ep = 2 J fp = 100 Hz Pa = 200 W
b)
Fig. 8. Effect of pulse power in case of constant average power and pulse energy: (a) on penetration depth; (b) on maximal tearing force
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tp = 2 ms Pa = 200 W
tp = 2 ms Pa = 200 W
a)
b)
Fig. 9. Effect of pulse power in case of constant pulse time and average power: (a) on penetration depth; (b) on maximal tearing force
In Fig. 10 and in Fig. 11 the effect of pulse energy on the penetration and maximal tearing force can be seen, while the pulse power was nearly constant, P p = 4000 W ± 400 W. Pulse energy was changed between 1 J and 40 J, pulse time was changed between 0.52 ms and 9.9 ms and pulse frequency was changed between 5 Hz and 200 Hz. In the diagram, the changes of pulse time are given.
Pp = 4,4 kW ± 0,4 kW Pa = 200 W
Fig. 10. Effect of pulse energy on penetration at constant average power and pulse power
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
Pp = 4,4 kW ± 0,4 kW Pa = 200 W
Fig. 11. Effect of pulse energy on maximal tearing force at constant average power and pulse power
We can see from the diagrams in Fig. 8 to Fig. 11 that the changing of pulse energy, pulse time and pulse frequency do not have a significant effect on the values of penetration and maximal tearing force in the examined ranges, expect in the case of big pulse energies over 10 J in Fig. 10 and 11. In this phenomenon the relative large laser spot (5 mm) may play a role: the energy density is low which can blunt the effect of the different settings. In this mentioned range the selection of laser settings is neutral from the viewpoint of technology, so the parameters can be optimized according to the maintenance of the laser device. In Fig. 10 and Fig. 11 we can see, that the penetration and the maximal tearing force decreases, the bond becomes weaker at constant average power and pulse power. The changing tendency of force and penetration suggests that the temperature is lower on the steel surface and that the plastic material does not tolerate large energy pulses. Since there are no differences visible in the vicinity of the joint, and the bubble formation is almost the same in all cases, the exact explanation needs further investigation. 4. Conclusions From this research the following can be concluded: the joining is feasible, and a longer heating time results in a deeper penetration and a higher tearing force, during the joining process bubbles are forming, the bubble formation is more intensive if the heating is longer. The bubbles weaken the joint because they behave as material continuity failures. The area of this bubbles can be controlled with the pressure value during the joining: higher pressure decreases the bubble formation, the tearing force can be increased effectively by increasing the surface roughness, which can be explained by the mechanical interlocking and higher laser adsorption,
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increasing the average laser power the penetration depth and the maximal tearing force were increased at constant heating time, the different laser pulse settings - at the same average laser power – have no significant effect on the penetration depth and on maximal tearing force in the lower pulse energy range (less than 10 J), but at settings in a higher pulse energy region the penetration depth and maximal tearing force are reduced.
Acknowledgements This work is connected to the scientific program of the “Development of quality oriented and harmonized R+D+I strategy and functional model at BME” project. These projects are supported by the New Széchenyi Development Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). References [1]
S. Katayama, Y. Kawahito: Laser direct joining of metal and plastic, Scripta Materialia 59 (2008) 1247–1250.
[2]
Jordan Rotheiser: Joining of plastics: Hanser Publisher, Munich, 1999.
[3]
A. Roesner, S. Scheik, A. Olowinsky, A. Gillner, U. Reisgen, M. Schleser: Laser Assisted Joining of Plastic Metal
[4]
F. G. Bachmann, U. A. Russek: Laser Welding of Polymers Using High Power Diode Lasers, Laser Processing of
[5]
G. W. Ehrenstein: Handbuch Kunststoff-Verbindungstechnik, Karl Hanser Verlag, München, 2004.
Hybrids, Physics Procedia 12 (2011) 373–380. Advanced Materials and Laser Microtechnologies (Proceedings Volume 5121) pp.385-818, 2003.
Physics Procedia 39 (2012) 108 – 116
LANE 2012
Laser assisted joining of metal pins and thin plastic sheets Andor Bauernhuber , Tamás Markovits Budapest University of Technology and Economics, Department of Vehicles Manufacturing and Repairing, 1111 Budapest, Bertalan Lajos u. 2, Hungary
Abstract The joining of plastics and metals in order to produce lightweight but robust parts has become more significant in the process of vehicle manufacturing. In the course of this study, the authors joined PMMA plastic and steel in a pin to plate geometry by pulsed Nd:YAG laser. Tensile tests were carried out to investigate the effects of heating time, the laser settings, the surface roughness and the clamping force on the joining strength. Experimental results showed that the joining is feasible and using adequate settings the strength can be optimised. © © 2012 2011 Published Published by by Elsevier Elsevier B.V. Ltd. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH Keywords: laser; steel; PMMA; hybrid; joining
1. Introduction Growing demand for new material combinations from the side of the industry, like plastic and metal combinations, requires new methods, also in the area of joining technologies. The fact that plastics are well applicable in technical structures accelerates the degree of their utilization, thus it is necessary to elaborate the procedure of their joining to other structural materials, like metals. The technologies applied so far have many disadvantages: in case of mechanical fasteners the stress peak around the joint and the difficult automation, while in case of the most widely used adhesives the long bonding time and the harmful volatile compounds cause difficulties [1, 2]. A possible solution for the listed problems may be provided by applying the laser beam directly to join metal and plastic, like it is already used to join the same kind of materials (metal to metal, plastic to plastic) [3, 4]. The aim of the research was to investigate the factors influencing the bond properties, and to describe their effect on the joint strength in pin to plate geometry. It was investigated how the heating time, the
* Corresponding author. Tel.: +36-146-31-838 . E-mail address: [email protected] .
1875-3892 © 2012 Published by Elsevier B.V. Selection and/or review under responsibility of Bayerisches Laserzentrum GmbH doi:10.1016/j.phpro.2012.10.020
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
clamping force, the surface roughness and the laser settings, like average power, pulse frequency, pulse time and pulse energy affect the joint strength and the penetration of the steel work piece into the plastic. 2. Experimental In the experiments, steel pin samples were bonded with plastic sheets. The steel material was S235, the plastic material was poly methyl metacrylate (Acriplex PMMA-XT), the geometry of steel sample and the schematic view of the experimental setup can be seen in Fig. 1, the size of the plastic sheet was 15 x 15 x 2 mm. This plastic was chosen because of its good strength results during the preliminary experiments and because of its transparency: the structural changes in the plastic and the connecting area could be observed easily. The laser beam source was a LASAG SLS 200 type, pulse mode Nd:YAG laser with a maximal pulse power of P max =5.5 kW and with an average power of P a =220 W. The power distribution of the laser beam was Gaussian (TEM 0,0 ).
Fig. 1. Schematic view of the experimental setup
The sample was irradiated from the plastic side. The plastic is highly transparent (90 %) to the laser beam [5], so the beam was transmitted through the plastic and was mainly absorbed by the steel surface. The steel was heated directly and the face surface transferred the heat to the plastic. The plastic became softer and finally melted due to the heat input. The softened material flowed back along the lateral surface of the pin and formed a burr ring at the entrance hole. At the end of the process, the steel pin penetrated into the sheet and was surrounded by a burr ring. After cooling, the joint was created. Investigating the heating time, it was changed from 3 s to 7 s, the applied laser settings were the following: f = 100 Hz, tp = 0,5 ms, E = 2 J where f is the pulse frequency, tp is the pulse duration in time, E is the pulse energy, the value of clamping force was 3.2 N. To examine the compressive force, the force values were 3.2 N, 6 N and 9.2 N. The heating time was 4 s. Investigating the surface roughness, the heating time was 6 s. The average surface roughness changed from R a =0,5 μm to R a =10 μm. Examining the laser settings, the heating time was 4s; the value of the clamping force was 3.2 N. The applied laser settings were the following: the frequency was changed between 4 Hz and 200 Hz, pulse time was varied between 0.3 ms and 9.9 ms, and the pulse energy was changed between 1 J and 40 J. In the first case, the effect of average power was investigated, the average power was varied from 120 W to 200 W by adjusting the pulse frequency. In the second case, constant pulse energy of 2 J, in the third case nearly constant pulse power of 4000 W ± 400W, in the fourth case constant pulse time of 2 ms was applied. The average power was 200 W in the last three cases. In each case 4.75 l/min argon shielding gas was applied and the average surface roughness of the steel pins on the lateral surface was altered between 0.5 μm and 2.5 μm except at investigating the effect of surface roughness. The pins were manufactured by turning. Before the experiment the steel pins were cleaned with acetone.
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To investigate the bonding force, the workpieces were torn 24 hours later. In the results section the force values are given because the determination of the bonding area was not obvious. Therefore the maximal tearing force was used to characterise the strength of the joint. 3. Results and Discussion 3.1. Effect of heating time and clamping force First, the effect of heating time (t h ) on the penetration depth and the tearing force was investigated. Our aim was to determine the process window in which the joint is feasible. The shortest time when a joint could be created was 3 s. A shorter heating time did not ensure the needed heat amount to melt the plastic, there was not a sufficient penetration. The longest time was 7 seconds, because at a longer heating time the steel pin perforated the plastic sheet, so that the measured penetration became bigger than 2 mm. Therefore the applied heating time values were 3, 4, 5, 6 and 7 seconds. In Fig. 2 (a) and (b), the photo of side and the top view of a joint, in Fig. 3 (a) and (b), the effect of heating time on penetration depth and maximal tearing force are shown at 3 different clamping force values.
F clamping = 3.2 N a)
F clamping = 6 N b)
F clamping = 9.2 N
Fig. 2. Photos of the joint at different clamping force values: (a) side view; (b) top view of the created joint and bubble formation
a)
b)
Fig. 3. Effect of heating time at different clamping force: (a) on penetration; (b) on maximal tearing force
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
In Fig. 3 (a) it can be seen, that in the case of a longer heating time the penetration increases: the temperature of the plastic rises in the vicinity of the steel, and the material softens. The softened material flows back next to the steel surface so that continually new plastic material melts in front of the heated pin. Accordingly, the longer the heating time, the deeper the steel pin penetration into the plastic due to the operating clamping force. The maximal tearing force can be seen in Fig. 3(b) against the heating time. The increasing rate of tear force shows a similar tendency with the results in Fig. 3 (a). To increase the maximal tearing force the joint has to be heated longer: the average tear force value changes from 150 N to 230 N. The explanation of the phenomenon is the deeper penetration: the contact surface grows in the case of a longer heating, resulting in a higher maximum force. If we compare the results of Fig. 3 (a) and Fig. 3 (b) at 3.2 N clamping force, we can see that the growth rate of the maximal tearing force has a slowing tendency with the heating time while the penetration grows in an accelerating way, creating a larger contact surface. The difference can be explained by the formation of bubbles: at longer heating times the temperature of the melted plastic increases, and it probably also decomposes, forming many bubbles at the steel surface. The bubbles weaken the joint because they separate the base plastic material from the steel. The bubble formation is more intensive in the case of longer heating times so their effect occurs primarily at the longer heating time range (Fig. 4 (a)). The effect of bubbles can be observed after the tearing of the joints as well. At low heating times the tearing occurs in many cases in the base plastic material and large parts of plastic remain on the face surface. At long heating times only a thin layer of plastic remains on the surface of the pin including the imprints of the created bubbles. The effect of heating time on the bubble formation and the surface of the pins after tearing can be seen in Fig. 4.
t heating = 3 s
a)
t heating = 7 s
t heating = 4 s
b)
t heating = 7 s
Fig. 4. Effect of heating time (a) on bubble formation; (b) on type of tearing
In Fig. 3 (a) and in Fig. 3 (b) the effect of clamping force on the measured penetration and maximal tearing force is plotted as well. As expected, the higher clamping force causes deeper penetration, proportional to the force value: the character of the curves is the same, they run in parallel. At clamping forces of 6 N and 9.2 N the maximal heating time is only 6 s, because the pin perforates the plastic sheet already at 7 s of heating time. The evolution of the curves in Fig. 3 (b) is not as obvious as in Fig.3 (a). The curves measured at different clamping forces are moving together at low heating times of 3 s and 4 s. At longer heating the curves split, and the higher clamping force leads to a higher tearing force. The phenomenon can be explained with the bubble formation again. Comparing the size and amount of bubbles at different clamping forces, we can see, that the bubble formation is less intensive at higher clamping forces: the higher pressure hinder the growing of bubbles and pushes them into the burr, as showed in Fig. 2 (b). Therefore the area of the bubbles can be regulated with the clamping force value and the result is a better contact and a higher tearing force. However, if too high pressure is used, the plastic sheet can be perforated without reaching the temperature needed for a good adhesion.
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3.2. Effect of surface roughness The effect of surface roughness on the maximal tearing force and on the penetration is showed in Fig. 5 (a) and (b). In this case a heating time of 6 s was chosen to reach a higher penetration, and to ensure a larger contact area on the lateral surface to. The tearing force value can be enhanced strongly by increasing surface roughness. On the one hand the soft and molten plastic takes the shape and flows into the valleys of the steel surface generating a mechanical interlocking. On the other hand the higher surface roughness improves the absorption of laser radiation at the top surface. This means a higher temperature at the same heating time, and so this leads to a deeper penetration. This increased penetration allows a larger contact area and makes possible for more roughness peaks to be in contact with the plastic material. The characteristic cross sections of the joints can be seen in Fig. 6: the roughness peaks and the bubbles on the surface are well identifiable.
a)
b)
Fig. 5. Effect of surface roughness (a) on penetration depth; (b) on maximal tearing force
a) Ra = 1 μm
b) Ra = 10 μm
Fig. 6. Cross section of the created joint at lateral surface (a) Ra=1 μm; (b) Ra=10 μm
3.3. Effect of laser settings In Fig. 7 (a) and (b) we can see the effect of average laser power on the penetration and on the tear force values. The power was adjusted by using different pulse frequencies while pulse energy and pulse time was unchanged. In both cases a similar tendency can be discovered as in Fig. 3 (a) and (b): the average power has an effect like the heating time. At least a power of 120 W is needed to create a joint. With increasing average power the penetration grows linearly, while the tear force has a slowing growth tendency. The difference is caused by the bubble formation, as mentioned in the section describing the heating time. In Fig. 8 (a) and (b) we can see the effect of pulse power on the penetration and maximal tearing force, while the pulse energy and pulse frequency was constant, E p = 2 J and f p = 100 Hz, respectively. Pulse
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
power was changed between 4.46 kW and 0.64 kW and pulse time was changed between 0.45 ms and 3.14 ms. In the diagram, the value of pulse time is given at each setting. In Fig. 9 (a) and (b) the effect of pulse power on the penetration and maximal tearing force is showed, while the pulse time was constant, t p =2 ms, and the pulse energy and pulse frequency was adjusted. Pulse energy was changed between 2 J and 10 J, pulse power was changed between 4.7 kW and 1 kW and pulse frequency was changed between 22 Hz and 100 Hz. In the diagram, the value of pulse energy is given at each setting.
a)
b)
Fig. 7. Effect of average power: (a) on penetration depth; (b) on maximal tearing force
Ep = 2 J fp = 100 Hz Pa = 200 W
a)
Ep = 2 J fp = 100 Hz Pa = 200 W
b)
Fig. 8. Effect of pulse power in case of constant average power and pulse energy: (a) on penetration depth; (b) on maximal tearing force
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tp = 2 ms Pa = 200 W
tp = 2 ms Pa = 200 W
a)
b)
Fig. 9. Effect of pulse power in case of constant pulse time and average power: (a) on penetration depth; (b) on maximal tearing force
In Fig. 10 and in Fig. 11 the effect of pulse energy on the penetration and maximal tearing force can be seen, while the pulse power was nearly constant, P p = 4000 W ± 400 W. Pulse energy was changed between 1 J and 40 J, pulse time was changed between 0.52 ms and 9.9 ms and pulse frequency was changed between 5 Hz and 200 Hz. In the diagram, the changes of pulse time are given.
Pp = 4,4 kW ± 0,4 kW Pa = 200 W
Fig. 10. Effect of pulse energy on penetration at constant average power and pulse power
Andor Bauernhuber and Tamás Markovits / Physics Procedia 39 (2012) 108 – 116
Pp = 4,4 kW ± 0,4 kW Pa = 200 W
Fig. 11. Effect of pulse energy on maximal tearing force at constant average power and pulse power
We can see from the diagrams in Fig. 8 to Fig. 11 that the changing of pulse energy, pulse time and pulse frequency do not have a significant effect on the values of penetration and maximal tearing force in the examined ranges, expect in the case of big pulse energies over 10 J in Fig. 10 and 11. In this phenomenon the relative large laser spot (5 mm) may play a role: the energy density is low which can blunt the effect of the different settings. In this mentioned range the selection of laser settings is neutral from the viewpoint of technology, so the parameters can be optimized according to the maintenance of the laser device. In Fig. 10 and Fig. 11 we can see, that the penetration and the maximal tearing force decreases, the bond becomes weaker at constant average power and pulse power. The changing tendency of force and penetration suggests that the temperature is lower on the steel surface and that the plastic material does not tolerate large energy pulses. Since there are no differences visible in the vicinity of the joint, and the bubble formation is almost the same in all cases, the exact explanation needs further investigation. 4. Conclusions From this research the following can be concluded: the joining is feasible, and a longer heating time results in a deeper penetration and a higher tearing force, during the joining process bubbles are forming, the bubble formation is more intensive if the heating is longer. The bubbles weaken the joint because they behave as material continuity failures. The area of this bubbles can be controlled with the pressure value during the joining: higher pressure decreases the bubble formation, the tearing force can be increased effectively by increasing the surface roughness, which can be explained by the mechanical interlocking and higher laser adsorption,
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increasing the average laser power the penetration depth and the maximal tearing force were increased at constant heating time, the different laser pulse settings - at the same average laser power – have no significant effect on the penetration depth and on maximal tearing force in the lower pulse energy range (less than 10 J), but at settings in a higher pulse energy region the penetration depth and maximal tearing force are reduced.
Acknowledgements This work is connected to the scientific program of the “Development of quality oriented and harmonized R+D+I strategy and functional model at BME” project. These projects are supported by the New Széchenyi Development Plan (Project ID: TÁMOP-4.2.1/B-09/1/KMR-2010-0002). References [1]
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